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Published in Crop Sci. 44:567-574 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

TURFGRASS SCIENCE

Nitrate Uptake of Seedling and Mature Kentucky Bluegrass Plants

Zhongchun Jiang*,a and W. Michael Sullivanb

a Dep. of Plant Sci., State Univ. of New York, College of Agriculture and Technology, Cobleskill, NY 12043
b Dep. of Plant Sci., Univ. of Rhode Island, Kingston, RI 02881

* Corresponding author (jiangz{at}cobleskill.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Understanding the relationship between seedling nitrate uptake rate (NUR) and mature plant NUR will benefit the selection of turfgrass cultivars for high NUR. Seedling NUR as affected by initial mowing is particularly important during turfgrass establishment. The primary objectives of this study were to determine NUR of individual Kentucky bluegrass (Poa pratensis L.) seedlings, to correlate seedling morphological characteristics with NUR, and to ascertain the relationship between seedling NUR and mature plant NUR. Thirty days after seeding, roots of seedlings were placed individually in a polystyrene cup filled with 2 mL of a nutrient solution containing 0.6 mM nitrate, which was replaced at 24-h intervals for 20 d. After the total leaf and total root length and area were determined, each seedling was planted in silica sand and maintained as a miniature turf for five additional months before NUR was determined in a 130-mL growth vessel. Seedling NUR was 10 to 60 nmol h–1 per seedling, decreased sharply to <10 nmol h–1 per seedling following the first clipping to remove 1/3 of the leaf blades by length. One week after the first clipping, NUR gradually increased to levels above 10 nmol h–1 per seedling. After seedlings were placed in deionized water for 24 h, NUR during the subsequent 24 h was stimulated, but the magnitude of stimulation differed among cultivars. We found significant among-cultivar and within-cultivar differences in seedling NUR, and significant and positive correlations between seedling NUR and seedling size. At the whole plant level, the correlation between seedling NUR and mature plant NUR was positive but generally not significant at P = 0.05.

Abbreviations: NUR, nitrate uptake rate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
IN MATURE turfgrass plants that were clipped regularly, NUR has been positively correlated with belowground morphological characteristics such as fibrous root length (Sullivan et al., 2000). Significant cultivar differences in mature plant NUR have been demonstrated for Kentucky bluegrass (Jiang et al., 2000), perennial ryegrass (Lolium perenne L.), and creeping bentgrass [Agrostis stolonifera var. palustris (Huds.) Farw.] (Bushoven and Hull, 2001). In contrast, little is known about turfgrass seedling NUR, although adequate N fertilizer is generally applied to ensure rapid turfgrass establishment from seed. Seedling growth parameters such as leaf and root lengths exhibited intraspecific variation in perennial ryegrass (Turner et al., 2001) and Kentucky bluegrass (Yamamoto et al., 1997). These growth parameters likely influence nitrate uptake of turfgrass seedlings because N is required for normal growth during turfgrass establishment. Nitrate uptake by early-stage turfgrasses following seedling emergence should affect nitrate leaching potential during the establishment period as indicated in a study by Bushoven et al. (2000). They found that perennial ryegrass, cultivar Palmer III, could prevent nitrate leaching within 8 wk of reseeding on killed turf plots. Selecting genotypes that show early signs of greater nitrate uptake ability would benefit the turfgrass manager and the environment when nitrate leaching is concerned, particularly during the turfgrass establishment period.

Although cultivar differences in mature plant NUR have been demonstrated in major cool-season turfgrass species (Cisar et al., 1989; Liu et al., 1993; Jiang and Hull, 1998; Bushoven and Hull, 2001; Jiang et al., 2001), the relationship between NURs of seedling and mature turfgrass plants has not been ascertained. A seedling that absorbs nitrate at a higher rate may grow larger than one that absorbs nitrate at a lower rate if other resources are not limiting. This is because most nitrate absorbed by cool-season turfgrasses is translocated to the shoots (Jiang and Hull, 1999; Bushoven and Hull, 2001; Jiang et al., 2002), and nitrate in the shoots promotes shoot growth (Van Quy and Champigny, 1992). A large plant at a mature stage may require a high NUR to supply N for continued growth. If a significant, positive relationship in NUR can be established between seedling and mature plants, genotypes having high NUR can be selected at the seedling stage alone, with selection and development time shortened.

Conventional NUR measurement in turfgrasses grown in nutrient solution has used batches or cultures of plants (Bowman et al., 1989; Cisar et al., 1989; Liu et al., 1993; Jiang and Hull, 1998, 1999). This approach can evaluate cultivar differences but cannot be used to detect potential differences among individual plants within a cultivar or breeding population. Kentucky bluegrass is a facultative apomictic species and its progeny regularly segregate into apomictic and nonapomictic groups (Huff and Bara, 1993). The nonapomictic group is considered genetically aberrant and can have sexual hybridization (Barcaccia et al., 1997). Thus, significant differences in NUR likely exist among individuals of a Kentucky bluegrass cultivar or breeding population. To identify superior nitrate absorbers at an early developmental stage, a screening system is needed in which NURs of individual seedlings can be conveniently determined.

This study used a rapid flow analysis system to screen seedlings of six Kentucky bluegrass cultivars for NUR beginning at 30 d after seeding. The objectives were to (i) compare cultivars and individuals within a cultivar for NUR; (ii) investigate the effects of partial shoot removal and N deprivation on seedling NUR; (iii) establish the relationships between seedling morphological characteristics and seedling NUR; and (iv) ascertain the relationship between NURs at seedling and mature stages of the same individuals within a cultivar.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Six Kentucky bluegrass cultivars, Blacksburg, Barzan, Conni, Dawn, Eclipse, and Gnome, were used in this study. Nitrate uptake and belowground morphology of these cultivars were studied previously using tiller-generated mature plants (Sullivan et al., 2000). Seeds were sown on silica sand and wet with deionized water daily. Thirty days after seeding, six seedlings of uniform size from each cultivar were gently removed from the sand, roots were rinsed three times in a nitrate solution containing 0.6 mM potassium nitrate + 0.6 mM calcium sulfate solution, pH 6.0, and each seedling was transferred to a polystyrene cup. The cup is designed for the rapid flow analysis system (Alpkem Corp., Clackamas, OR) which analyzes solution nitrate concentration. Each cup was prefilled with 2 mL of the nitrate solution. A 14-h photoperiod was provided with an incandescent lamp producing 300 µmol m–2 s–1 photosynthetic photon flux density at the middle level of the leaves. After 24 h in the nitrate solution, the seedling was transferred to a new cup filled with 2 mL of fresh nitrate solution. The old cup containing the used nitrate solution was weighed to determine the final weight, which was used for calculating the final volume of the remaining solution in each cup. The old cup was then placed on the tray designed for the rapid flow analysis system and the final nitrate concentration of the residual nitrate solution was analyzed by the system.

The NUR of each seedling was monitored as above daily with a 0.6 mM potassium nitrate + 0.6 mM calcium sulfate solution for an initial 4 d to induce nitrate transporters in the roots. Any nitrate uptake during this period would depend on energy reserves within the seedling because most other essential nutrient elements were not supplied from the nitrate uptake solution. From Day 6 through Day 22, 1/4-strength Hoagland's nutrient solution with nitrate concentration modified to 0.6 mM using potassium nitrate was provided to continue the daily monitoring of nitrate uptake.

To mimic the effects of fluctuation in nitrate concentration and mowing on nitrate uptake, seedlings were subjected to two treatments during the course of daily monitoring: N deprivation and partial shoot removal. On Days 5 and 19, deionized water was provided in place of the nutrient solution to compare NUR before and NUR after nitrate supply was withheld. On Day 9, at 0900 h, 1/3 of leaf blade lengths were clipped off of each seedling for the first time and again on Day 22 at 0900 h to mimic the effects of mowing on nitrate uptake.

At the end of the seedling nitrate uptake experiment, seedlings with roots attached were scanned on a scanner (ScanJet 4c, Hewlett-Packard Co.) contained in a Delta-T SCAN splash protection system (Delta-T Devices Ltd., Cambridge, UK), with a black background. Total lengths and areas of roots and leaves of each seedling were determined from the images, as described previously (Sullivan et al., 2000). Leaves immediately before the first clipping were also scanned and analyzed for total length and area. Roots were not scanned before the first clipping to avoid mechanical shock, which would affect nitrate uptake during subsequent days.

After the nitrate uptake experiment, the seedlings were planted individually in polystyrene containers (3.8-cm diam., 21-cm depth; Stuewe & Sons, Inc., Corvallis, OR) filled with silica sand. The plants were maintained in a greenhouse for five additional months before they were excavated for nitrate uptake measurement. During this 5-mo maintenance period, plants were clipped weekly to approximately 5 cm and watered daily with a 1/2-strength Hoagland's nutrient solution except during weekends when tap water was provided. The plants received sunlight, supplemented with a sodium vapor lamp on cloudy days, providing approximately 800 µmol m–2 s–1 photosynthetic photon flux density at the canopy level.

At the end of the greenhouse growth period, each plant produced a minimum of five tillers and filled the container. These plants were excavated from sand, and roots and rhizomes were gently washed under running tap water to completely remove sand. Each plant with all its tillers was placed in a 130-mL growth vessel with the roots and rhizomes submerged in 120 mL of deionized water for 1 d. The NUR was monitored three times at 0.6 mM, followed by monitoring at 3.0 mM nitrate concentration for two 1-wk periods. The 0.6 mM nitrate concentration was used to evaluate nitrate uptake by the high-affinity nitrate transport system, and 3.0 mM was used to evaluate nitrate uptake by the low-affinity nitrate transport system (Forde, 2000; Louahlia et al., 2000). At the 0.6 mM concentration, nitrate uptake was determined during three consecutive periods in the following order: a 10-h light period (800 µmol m–2 s–1) corresponding to the day, a 10-h dark period corresponding to the night, and a full day (a 14-h light period). At the 3.0 mM nitrate concentration, NUR was determined during two 1-wk periods under 14/10-h light/dark cycles. The nutrient solution in each growth vessel was continuously aerated through an aeration line connected to an air pump via a manifold. The day and night air temperatures were 24 and 15°C, respectively.

The order of the seedlings or mature plants was randomized with respect to cultivars within each of six sets of seedlings. Statistical analysis was performed using the SAS System for Windows version 8 (SAS Institute, Inc., Cary, NC). Because ANOVA showed no significant interaction between observation and cultivar, all observations for the six seedlings of a cultivar within a certain period were used as replications for that cultivar. Significant means were separated by the LSD procedure and simple regression analysis was done within the SAS System. To correlate NUR with root and shoot morphological characteristics, we used the mean NUR values determined on 2 or 3 d immediately before the day when clippings were collected, so that the effect of seedling size on nitrate uptake would be reflected to the greatest extent.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The general trend in NUR during the seedling monitoring period is shown in Fig. 1 . From Days 1 through 8, NUR varied between 10 and 60 nmol h–1 per seedling. After the first clipping on Day 8 to remove 1/3 of the leaf blade length, NUR decreased dramatically, particularly on the second day following the first clipping, when no nitrate uptake but nitrate efflux was detected. Following one day of efflux, NUR gradually increased to the levels seen at the beginning of the experiment. Defoliation has been shown to cause an immediate NUR decrease in perennial ryegrass (Louahlia et al., 1999) and this decrease is probably a result of decreased shoot demand for N and nitrate accumulation in the roots (Gniazdowska and Rychter, 2000). Nitrate in the apoplast exterior to the Casparian strip of seedling roots may have contributed to the apparent nitrate efflux observed in this study.



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  		Fig. 1. Time course of seedling nitrate uptake of six Kentucky bluegrass cultivars: BL, Blacksburg; BZ, Barzan; CN, Conni; DN, Dawn; EC, Eclipse; GM, Gnome. Thirty days after seeding, six seedlings of uniform size from each cultivar were placed individually in polystyrene cups filled with 2 mL of a nutrient solution containing 0.6 mM nitrate, and the rate of nitrate uptake was monitored at 24-h intervals. On Days 5 and 19, deionized water was provided (indicated by "–NO3–") in place of the nitrate solution. Immediately before the start of nitrate uptake on Days 9 and 22, one-third of the leaf blade lengths were clipped off of each seedling (indicated by "1st cut" and "2nd cut"). On Day 10, no nitrate uptake but nitrate efflux from the roots was detected (indicated by "Efflux").

 
Cultivar differences in NUR were observed before the first clipping on Day 9 and 1 wk thereafter, from Days 16 to 21 (Table 1). Although cultivar differences were not consistent during the first four days (Fig. 1), the grand mean (n = 24) of NUR was highest in Gnome (Table 1). Conni had a lower NUR than Dawn, Eclipse, and Gnome during the initial 4 d. Thereafter, NUR of Conni was not significantly lower than other cultivars except Gnome. During the week (Days 19 through 15) following the first clipping and one day (Day 22) following the second clipping, no significant differences were observed among the six cultivars (Table 1). In a previous study involving the same cultivars, in which tiller-generated plants from a single mother plant were used for each cultivar, Blacksburg had the highest NUR and Gnome had the lowest NUR on a per-plant basis (Jiang et al., 2000). These inconsistent results indicate (i) that mature plants perform differently from seedlings with respect to NUR, and (ii) that individuals within a cultivar may differ in NUR.


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  		Table 1. Seedling nitrate uptake rate of six Kentucky bluegrass cultivars. Nitrate uptake of six seedlings per cultivar were individually monitored in a nutrient solution containing 0.6 mM nitrate at 24-h intervals. Nitrate uptake rate was determined each day from Day 1, when seedlings were at 30 d after seeding. On Days 5 and 19, deionized water was provided in place of the nutrient solution. Immediately before the start of nitrate uptake on Days 9 and 22, one-third of the leaf blade length was cut off of each seedling.

 
Although there was a general trend of increasing NUR from Days 2 to 4 and from Days 11 to 18 (Fig. 1), we observed no significant differences in NUR between any two consecutive days during these two periods at P = 0.05 (results of statistical analyses not shown). However, significant differences in NUR were observed between Days 4 and 6, and between Days 18 and 20 in some cultivars (Table 2). Nitrate deprivation on Days 5 and 19 may have increased seedling NUR of some cultivars on Days 6 and 20, respectively (Fig. 1, Table 2). Previous reports (Bowman et al., 1989; Jiang et al., 2000) have indicated that N deficiency or interruption in nitrate supply could result in higher NUR following these treatments. However, comparing NURs of the day before and the day after nitrate deprivation revealed that Blacksburg, Eclipse, and Gnome exhibited no significant differences in NUR between Days 4 and 6 at P = 0.05 (Table 2). Barzan, Conni, and Dawn exhibited no significant differences in NUR between Days 18 and 20 at P = 0.05. These results suggest that the degree to which NUR responds to an interruption in nitrate supply can differ, probably depending on previous conditions of the seedling, such as the balance between internal N reserves and N demands of the seedling. In addition, NUR decreased gradually in all cultivars following the NUR increases, except Gnome after the 1st peak of NUR (Fig. 1). As a result of the rapid increases in NUR following nitrate deprivation, seedlings of most cultivars likely exceeded their existing capacities to assimilate nitrate. Nitrate accumulation in these seedlings may have resulted in the gradual decrease in NUR due to feedback inhibition of the nitrate transport systems.


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  		Table 2. Effects of nitrate deprivation and partial shoot removal on seedling nitrate uptake rate of six Kentucky bluegrass cultivars. See Table 1 title as well as Materials and Methods section of the text for nitrate uptake measurement procedures.

 
Partial shoot removal on Day 9 at 0900 h and Day 22 at 0900 h decreased seedling NUR on Days 9 and 22, respectively (Fig. 1). Shoots are the major sites of nitrate assimilation in Kentucky bluegrass (Jiang and Hull, 1999) and partial removal of shoots would decrease the demand for N, thus reducing NUR. Decreased shoot demand for N could also lead to nitrate accumulation in the roots due to reduced translocation of nitrate to shoots. Increased nitrate accumulation in the roots would decrease NUR (Gniazdowska and Rychter, 2000). However, we observed that at P = 0.05, Gnome exhibited no significant difference in NUR between Days 8 and 9, and Conni exhibited no significant difference in NUR between Days 21 and 22 (Table 2), suggesting that NUR response to partial shoot removal varied with cultivars.

Significant differences in grand mean NUR were noted among individual seedlings within each of the six Kentucky bluegrass cultivars (Fig. 2) . For example, in Blacksburg, Seedling 4 had a higher NUR than Seedling 2 (Fig. 2A), and in Barzan, Seedling 1 was higher than other seedlings (Fig. 2B). In Gnome, Seedlings 3 and 6 were higher than Seedling 1 (Fig. 2F). Five individuals of Gnome had NUR values > 20 nmol h–1 per seedling, while most individuals of other cultivars had NUR values < 20 nmol h–1 per seedling. These observations demonstrate that genetic variation in NUR exists within the cultivar. Other studies (Huff and Bara, 1993; Barcaccia et al., 1997) have shown that seeded Kentucky bluegrass plants can produce apomictic progeny, which are genetically identical to the maternal genotype, and aberrant progeny, which differ from the maternal genotype and can have various genetic origins. The individual differences in NUR observed within each of the six cultivars may be due to the genetic differences because nitrate uptake is mediated by nitrate transporters in the roots (Crawford and Glass, 1998). Other conditions, such as seedling growth rate and root system size, may have also caused these differences.



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  		Fig. 2. Comparisons of nitrate uptake rate (NUR) among seedlings within each of six Kentucky bluegrass cultivars: (A) Blacksburg, (B) Barzan, (C) Conni, (D) Dawn, (E) Eclipse, (F) Gnome. The mean NUR for each seedling was based on 19 daily observations shown in Fig. 1. The LSD at P < 0.05 was shown as the last bar in each graph.

 
The total length and total area of the leaves and roots of the seedling differed among cultivars (Table 3), but leaves from the first clipping exhibited no significant differences among most of the cultivars. Gnome had a substantially greater total leaf length and area, more than 50% greater than those of other cultivars. Leaves from the second clipping represented regrowth following the first clipping and Gnome had a greater total leaf length than Conni and Eclipse. Gnome also exhibited greater total root length than Conni and Blacksburg (Table 3).


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  		Table 3. Morphological characteristics of seedling leaves and roots of six Kentucky bluegrass cultivars. At the time of the first clipping, seedlings were at 38 d after seeding, and at the time of the second clipping, seedlings were at 52 d after seeding. Root length and area were not determined at the time of the first clipping to avoid mechanical shock to roots.

 
Seedling NUR was correlated positively with total length and total area of leaves and roots of the seedlings (Fig. 3) . We used the mean NUR values determined on 2 or 3 d immediately before the day when clippings were collected, so that the effects of seedling size on NUR could be reflected to the greatest extent. Total leaf length (Fig. 3A) and total leaf area (Fig. 3B) of the first clippings were positively correlated with seedling NUR of Days 6 to 8, with R2 values of 0.966 and 0.995, respectively. These R2 values were higher than those for the correlations between total leaf length or area of the second clippings and seedling NUR of Days 20 and 21 (Fig. 3C and 3D). The R2 value for the correlation between total root length at the end of seedling nitrate uptake experiment period and NUR of Days 20 and 21 (Fig. 3E) was 0.682, lower than all other R2 values. In a previous study (Sullivan et al., 2000), we demonstrated that the total root length of tiller-generated mature Kentucky bluegrass plants was positively correlated with NUR, with an R2 value of 0.670. The present study confirmed that root size as measured by total root length or surface area had a positive effect on NUR, even in seedlings. In addition, shoot size as measured by total leaf length and area also had a significant, positive effect on seedling NUR (Fig. 3A–3D).



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  		Fig. 3. Relationships between nitrate uptake rate (NUR) and morphological characteristics of Kentucky bluegrass seedlings. (A) relationship between NUR of Days 6 to 8 and total leaf lengths of the first clippings, (B) relationship between NUR of Days 6 to 8 and total leaf area of the first clippings, (C) relationship between NUR of Days 20 and 21 and total leaf length of the second clippings, (D) relationship between NUR of Days 20 and 21 and total leaf area of the second clippings, (E) relationship between NUR of Days 20 and 21 and total root length of the second clippings, (F) relationship between NUR of Days 20 and 21 and total root surface area of the second clippings.

 
Five months after the plants were maintained in a greenhouse and clipped weekly, significant differences in NUR were observed among the six cultivars, with Gnome having a greater NUR than Dawn (Table 4). During the initial 10-h light period, which followed a day of deionized water, NUR at an external nitrate concentration of 0.6 mM was above 2 µmol h–1 per plant for most cultivars except Dawn. During the subsequent two monitoring periods, one corresponding to the dark and one to the light phase, NUR decreased to below 2 µmol h–1 per plant. The higher NUR during the initial monitoring period may have been a result of prior nitrate deprivation, which tended to increase NUR (Table 2; Bowman et al., 1989). The NURs during the night and during the day were not significantly different at P = 0.05 (ANOVA on data of day and night NURs resulted in P values ranging from 0.13 for Conni to 0.96 for Eclipse.) The observation is in agreement with an earlier report that Kentucky bluegrass could assimilate nitrate in the night at a rate comparable with rates observed during the day (Jiang and Hull, 2000). In 25-d-old cotton seedlings, NURs during the night were similar to those observed during the day (Aslam et al., 2001), which was attributed to a high energy availability at a night temperature of 30°C. In 9-d-old barley seedlings, NUR determined under light was not different from NUR determined under darkness, regardless of whether the plants were preilluminated or not (Sehtiya and Goyal, 2000). Kentucky bluegrass is a cool-season species and cool temperatures at night may have enhanced nitrate uptake in our experiment.


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  		Table 4. Mature plant nitrate uptake rate at 0.6 mM nitrate during a 10-h light period (Day 1), a 10-h dark period (night), and a 14-h light period (Day 2), and at 3.0 mM nitrate during two 1-wk periods with 14/10 h day/night cycles. NUR of Day 1 was determined 1 d after the plant roots were submerged in deionized water. Nutrient solution with 0.6 or 3.0 mM nitrate was replaced at the beginning of each period.

 
Nitrate uptake of the mature plants was monitored at an external nitrate concentration of 3 mM during two 1-wk periods (Table 4). At nitrate concentrations > 1 mM, nitrate uptake is regulated mainly by a low-affinity nitrate transport system, which has a linear relationship with external nitrate concentration. At nitrate concentrations < 1 mM, a high-affinity nitrate transport system plays a major role (Forde, 2000). We found no significant differences in NUR during Week 1, but during Week 2, Gnome had a greater NUR than Dawn (Table 4). The mean NUR was also higher in Gnome than in Dawn. Simple correlation analysis revealed that NUR at the 3 mM nitrate concentration was positively correlated with NUR at the 0.6 mM nitrate concentration, with an R2 value of 0.906. Previously, we demonstrated that NUR of Kentucky bluegrass increased with increasing external nitrate concentration within the range of 0.05 to 1.0 mM (Jiang and Hull, 1998). In the present study, we found that when determined at 3 mM across a week, NUR was not higher than that determined at 0.6 mM across a day or shorter periods (Table 4). When determined immediately after N deprivation, NUR at 0.6 mM nitrate concentration was even higher than at 3 mM. These observations indicate that the N status within the plant has a major influence on NUR.

Correlation analysis was performed between mean NURs during various periods at the seedling stage and mean NUR of the mature plants at 0.6 or 3.0 mM nitrate concentrations (Table 5). The relationships between NUR of the seedlings and NUR of the mature plants were generally not significant at P = 0.05 (Table 5). Mean seedling NUR of Days 6 to 8, the 3 d following the first N deprivation, was significantly (P = 0.047) correlated with mature plant NUR at 0.6 mM immediately following deionized water treatment. Seedling NUR of Days 16 to 18, approximately 7 d after the first clipping, was also significantly (P = 0.046) correlated with mature plant NUR at 0.6 mM immediately following deionized water treatment. These significant correlations appear to suggest that the inducible high-affinity nitrate transport system, which is induced by nitrate following a period of N deprivation and operates at nitrate concentrations < 1.0 mM (Forde, 2000), may perform consistently at the seedling and mature stages of a cultivar. The P values for the correlation coefficients between mature plant NUR at 3.0 mM and seedling NUR at 0.6 mM were greater than the P values for the correlation coefficients between mature plant NUR at 0.6 mM and seedling NUR at 0.6 mM, suggesting that the major nitrate transport system was different at the two nitrate concentrations.


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  		Table 5. Correlations between seedling and mature plant nitrate uptake rate (NUR, nmol h–1 per seedling or µmol h–1 per mature plant) in Kentucky bluegrass cultivars. See Tables 1 and 4 as well as Materials and Methods section of the text for nitrate uptake measurement procedures.

 
Taken together, our data indicate that at the whole plant level, seedling NUR may not be used to predict mature plant NUR in all circumstances. The inherent ability of a cultivar to produce more tillers, fibrous roots, and rhizomes during the mature stage can increase the mature plants' nitrate uptake ability because these characteristics have been found to affect NUR of mature plants (Sullivan et al., 2000).

In summary, we observed significant differences in NUR among six Kentucky bluegrass cultivars at the seedling and the mature stage. Significant differences among individual seedlings within each of the six cultivars were also observed. Cultivar differences in mature plant NUR were consistent at 0.6 and 3.0 mM nitrate concentrations. When seedlings were supplied with 0.6 mM nitrate and then nitrate supply was withheld for 1 d, NUR during the following day was stimulated, and the magnitude of stimulation differed among cultivars. When seedling leaves were clipped to remove 1/3 of leaf blade lengths, NUR decreased sharply and immediately, but approximately 1 wk later, NUR increased to the level before the first clipping. The total leaf length, total leaf area, total root length, and total root surface of the seedling were all positively and significantly (P = 0.05) correlated with seedling NUR. At the whole plant level, the correlation between seedling NUR and mature plant NUR was positive but generally not significant at P = 0.05. Other characteristics of the mature plants, such as the ability to produce tillers and rhizomes, may have contributed to the lack of correlation between seedling NUR and mature plant NUR.

Received for publication June 28, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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